1. Field of the Invention
The present invention relates to a spark plug.
2. Description of the Related Art
In a direct injection type gasoline engine (generally called a xe2x80x9cdirect injection enginexe2x80x9d) which has been put into practical use in recent years, since gasoline is injected into the engine, an air-fuel mixture readily comes into direct contact with the spark plug. Therefore, substances resulting from incomplete combustion (hereinafter referred to as xe2x80x9cuncombusted substancesxe2x80x9d), such as carbon and uncombusted fuel, accumulate on the spark plug. Such accumulation occurs specifically on the tip end surface of an insulator which fixedly holds a center electrode and on a circumferential surface of the insulator located inside a metallic shell, with the result that smoking occurs in the spark plug. Further, even in a conventional gasoline engine, smoking occurs in a spark plug when the engine is started at a very low temperature; e.g., at xe2x88x9210xc2x0 C. or lower, in an extremely cold environment.
For example, a conventional surface discharge plug as shown in FIG. 13 which is configured such that spark is produced between a ground electrode 4 and a center electrode 2 and such that at least a portion of the spark travels along the surface of the insulator 3 causing problems at low temperature. At low temperature, an air-fuel mixture condenses into fuel droplets and water droplets (liquid droplets) F, which then enter the space between a metallic shell 5 and the insulator 3. Such liquid droplets flow down along the surface portion (circumferential surface) 3c of the insulator 3, and may remain at the tip end portion (lowest portion) of the insulator 3 due to their viscosity. Some carbon particles C adhering to the surface portion 3c of the insulator 3 flow down, passing over the liquid droplets F. In such a case, due to inverter voltage remaining in the center electrode 2, the carbon particles C are aligned in a row between the tip end portion 3a of the insulator 3 and the tip end portion 4a of the ground electrode 4. When volatile components of the liquid droplets F evaporate, only the carbon particles C remain, in the form of a bridge, so that the insulation resistance of the insulator 3 decreases. As a result, sparks are not produced properly at the spark discharge gap g between the center electrode 2 and the ground electrode 4, with the result that engine starting performance at low temperature deteriorates.
Meanwhile, when a spark plug is used for a long period of time in a low-temperature environment such that the electrode temperature of the spark plug becomes 450xc2x0 C. or lower, a phenomenon called smoking contamination occurs easily. The term xe2x80x9csmoking contaminationxe2x80x9d refers to a phenomenon wherein the surface portion 3c of the insulator 3 is covered by electrically conductive contaminants such as carbon C with a resultant decrease in insulation resistance, and therefore spark tends to occur at locations other than the spark discharge gap g; e.g., spark (deep spark) occurs at the side of the base end portion of the metallic shell 5 along the surface portion 3c of the insulator 3, with resultant failure in operation. In order to prevent smoking contamination, in some cases, a spark plug is attached to a cylinder head 1 such that the tip end 3a of the insulator 3 projects into a combustion chamber 1b from a combustion chamber wall 1a of the cylinder head 1. In such a case, the insulator 3 is exposed directly to combustion gas, so that the tip end temperature of the spark plug increases, and electrically conductive contaminants such as carbon are combusted with ease by means of a self-cleaning effect. However, the angle of advance ignition at which pre-ignition occurs (hereinafter referred to as xe2x80x9cpre-ignition occurrence anglexe2x80x9d) tends to decrease, with a resultant decrease in heat resistance.
The present invention generally provides a spark plug comprising a cylindrical metallic shell having a stepped portion on an inner wall thereof; an insulator disposed inside the metallic shell while being engaged with the stepped portion of the metallic shell and having an axially extending through hole; a center electrode fixed within the through hole of the insulator such that a tip end portion of he center electrode projects from the tip end of the insulator or is located at the tip end; and a ground electrode having a base end portion connected to the tip end portion of the metallic shell and a tip end portion bent toward the center electrode to thereby form a spark discharge gap in cooperation with a side surface of the center electrode.
The present invention can be applied not only to spark plugs (such as surface discharge spark plugs and multi-electrode spark plugs) in which spark discharge occurs between the tip end surface of the ground electrode and the side surface of the center electrode, but also to spark plugs (such as parallel-type spark plugs) in which spark discharge occurs between the side surface of the ground electrode and the tip end surface of the center electrode.
According to a first aspect of the present invention, the insulator is formed such that the outer diameter of the insulator decreases toward the tip end side from an engagement position at which the insulator engages the stepped portion and such that the diameter decreases stepwise at an axial position between the engagement position and the tip end of the insulator; and a diameter reduction ratio Y1=D1/d1 is 0.6 or less in a region of at least 2 mm extending from the tip end surface of the insulator toward the base end side, wherein D1 represents the outer diameter of the insulator measured at an arbitrarily determined axial position, and d1 represents the inner diameter of the tip end portion of the metallic shell.
In the spark plug according to the first aspect, since the insulator has a stepped portion, a large space can be secured between the insulator and the metallic shell. Accordingly, fuel and water hardly remain in that space, whereby formation of a bridge of carbon atoms is prevented. Thus, low temperature starting performance does not deteriorate. Further, since the diameter reduction ratio Y1=D1/d1 is 0.6 or less in a region of at least 2 mm extending from the tip end surface of the insulator toward the base end side, a large space can be secured between the insulator and the metallic shell. Therefore, the cooling effect achieved by means of fresh air-fuel mixture is enhanced, so that the temperature increase at the tip end of the spark plug is mitigated even though the tip end portion of the insulator projects into the combustion chamber of the engine. Accordingly, the pre-ignition occurrence angle can be increased, and thus heat resistance can be improved. Moreover, the strength of electric field increases at the stepped portion as compared with other portions. Therefore, even when spark discharge occurs between the circumferential surface of the insulator and the inner wall of the metallic shell, the spark discharge occurs predominantly at the stepped portion, so that spark discharge at the base end side of the metallic shell can be prevented, and a self-cleaning effect provided by spark discharge is enhanced further. Accordingly, high insulation resistance of the insulator can be maintained and smoking contamination hardly occurs.
According to a second aspect of the present invention, the insulator is formed such that the outer diameter of the insulator decreases toward the tip end side from an engagement position at which the insulator engages the stepped portion and such that the diameter decreases stepwise at an axial position between the engagement position and the tip end of the insulator; and a clearance ratio Y2=(d1xe2x88x92D1)/d1 is 0.4 or greater in a region of at least 1 mm extending from the tip end surface of the metallic shell toward the base end side, wherein D1 represents the outer diameter of the insulator measured at an arbitrarily determined axial position, and d1 represents the inner diameter of the tip end portion of the metallic shell.
In the spark plug according to the second aspect, since the insulator has a stepped portion, the tapered portion of the insulator has a stepped portion and the clearance ratio Y2=(d1xe2x88x92D1)/d1 is 0.4 or greater in a region of at least 1 mm extending from the tip end surface of the metallic shell toward the base end side. Therefore, a larger space can be secured between the insulator and the metallic shell. Accordingly, fuel and water hardly remain in that space, whereby formation of a bridge of carbon atoms is prevented. Thus, low temperature starting performance does not deteriorate. Moreover, the strength of electric field increases at the stepped portion as compared with the remaining portion. Therefore, spark discharge at the base end side of the metallic shell can be prevented and a self-cleaning effect provided by spark discharge is enhanced further. Accordingly, high insulation resistance of the insulator can be maintained and smoking contamination hardly occurs.
In the spark plugs of the first and second aspects, when a distance in the radial direction between the tip end surface of the ground electrode and an intersection between a line axially extending from the circumferential surface of the insulator and a line radially extending from the tip end surface of the insulator is defined to be an overlap amount X, the overlap amount X is preferably set to be greater than xe2x88x920.5 mm but not greater than 0.1 mm. In this case, fuel droplets and water droplets which are produced as a result of condensation of a fuel-air mixture at low temperature and flow down along the surface portion of the insulator encounter difficulty in remaining at the tip end portion (lowest portion) of the insulator, so that formation of a bridge of carbon particles is suppressed. Therefore, starting performance at low temperature is improved.
According to a third aspect of the present invention, when a distance in the radial direction between the tip end surface of the ground electrode and an intersection between a line axially extending from the circumferential surface of the insulator and a line radially extending from the tip end surface of the insulator is defined to be an overlap amount X, the overlap amount X is set to be greater than 0 mm but not greater than 0.1 mm.
In the spark plug of the third aspect, fuel droplets and water droplets which are produced as a result of condensation of an air-fuel mixture at low temperature and flow down along the surface portion of the insulator encounter difficulty in remaining at the tip end portion (lowest portion) of the insulator, so that formation of a bridge of carbon particles is suppressed. Therefore, starting performance at low temperature is improved.
In the spark plug of the third aspect, the insulator being preferably formed such that the outer diameter of the insulator decreases toward the tip end side from an engagement position at which the insulator engages the stepped portion and such that the diameter decreases stepwise at an axial position between the engagement position and the tip end of the insulator. In this case, as in the spark plugs of the first and second aspects, the spark discharge occurs predominantly at the stepped portion, so that spark discharge at the base end side of the metallic shell can be prevented and a self-cleaning effect provided by spark discharge is enhanced further. Accordingly, high insulation resistance of the insulator can be maintained and smoking contamination hardly occurs. Moreover, the pre-ignition occurrence angle can be increased and thus heat resistance can be improved.
Preferably, when the spark plug is attached to the cylinder head of an engine, the tip end portion of the metallic shell projects from a combustion chamber wall toward a combustion chamber and the projection amount L2 is at least 1 mm. In this case, entry of fuel and water into the space between the tip end portion of the metallic shell and the tip end portion of the insulator is suppressed, so that occurrence of bridging at the tip end surface of the metallic shell is prevented.
Preferably, the metallic shell has a substantially constant inner diameter over an area extending between the stepped portion and the tip end portion. In this case, since the inner diameter of the metallic shell can be made relatively small, entry of carbon particles and the like into the space between the tip end portion of the metallic shell and the tip end portion of the insulator is suppressed, whereby smoking contamination is prevented. Further, since the stepped portion formed on the inner wall of the metallic shell has no edge portion, spark discharge at the base end side of the metallic shell can be reduced.
Thus it is an object of the present invention is to provide a spark plug which has excellent low temperature starting performance, heat resistance, and contamination resistance, and which prevents formation of a bridge of carbon particles.